This invention relates to an alignment apparatus such as a mask-to-wafer alignment system for photolithography in fabricating semiconductor devices.
SPIE Vol. 773, Electron-Beam, X-Ray and Ion-Beam Lithographies VI (1987), pages 7-14, discloses a new interferometric optical-heterodyne method which has been developed for detection of displacement between a mask and a wafer. This prior-art method uses three symmetrically-arranged gratings and a coherent light consisting of two orthogonally-polarized components with different frequencies. The displacement between the mask and the wafer is detected from the phases of beat signals. According to the prior-art method, using a 0.76-micrometer-period-grating system and a He-Ne transverse-mode Zeeman laser (wave length=0.6328 micrometers), sensitivity better than 1.degree./0.01 micrometers was obtained and displacement smaller than 5 nm was detected independently of the mask-wafer gap variations.